A variety of medical devices exist that are implanted inside the body and undergo a dimensional change. For example, a bone lengthening device is one type of implantable device that is typically inserted into first and second portions of a severed or broken bone. The device is then periodically lengthened to distract or grow the bone over a period of time. Such adjustments made to the bone lengthening device may be invasive or even non-invasive. As another example, growing rods or distraction devices may be secured to a subject's spine. These devices may be used to correct a medical condition such as scoliosis. In still other applications, these devices may be used to increase the distance between adjacent vertebrae to reduce symptoms associated with lumbar spinal stenosis or pinched nerves. Other bones such as the jaw bone may include an implantable medical device that is configured to elongate over time.
Regardless of the nature in which the implanted medical device is used, there often is a need to determine the length of the implant as it exists inside the patient at any given moment. As an example, after the implanted medical device has undergone a length adjustment there is a need to determine whether or not the desired quantity of lengthening was indeed achieved. U.S. Patent Application Publication No. 2010/0094302 discloses a non-invasive medical implant device that uses microphone sensor on an external adjustment device to sense when an internally-located magnet is undergoing rotation. Specifically, the microphone sensor picks up an acoustic signal (e.g., click) that is periodically generated by rotation of an internal magnet that is part of the implantable medical device. By counting the number of clicks, the external adjustment device can then translate this into an estimated length of the device.
In implanted medical devices that utilize an internally located magnet to effectuate a change in length or force, there sometimes exists the state of “stalled distraction.” Stalled distraction refers to the phenomenon that occurs when the implant magnet (i.e., the magnet located within the device implanted inside the body) ceases complete rotations and there is a slipping in the magnetic coupling between the magnetic field(s) of the implant magnet and the externally applied magnetic field(s). This can occur, for example, when the compressive force on the implant drive mechanism exceeds the available distraction force provided by the torque coupling of the externally applied magnetic field (either by an electromagnet or permanent magnet) to the implant magnet. In such instances, while the external magnetic field may be rotating, the internal magnet contained within the implanted device may be prevented from rotating. This may result in inaccurate measurements of the implanted device. For example, the length of the implant may be based on the number of rotations of an externally applied magnetic field which is based on the assumption that the internal magnet rotates in a corresponding manner. If magnetic coupling between the internal magnet and the externally applied magnetic field is interrupted due to slippage, one may not know the actual length of the implant because the internal magnet failed to rotate in accordance with the externally applied magnetic field. The externally applied magnetic field (e.g., five rotations) may lead one to estimate an implant length that is larger than the actual implant length because the internal magnet slipped and failed to rotate in 1:1 correspondence with the externally applied magnetic field (e.g., internally located magnet only rotated three times).